Tuesday, September 11, 2012

My research focuses on two populations of sea turtles found in the East Pacific, the East Pacific Green Sea Turtles (species: Chelonia mydas) and the North Pacific Loggerhead Sea Turtles (species: Caretta caretta). Here's a bit more information on these two populations which are the focus of my studies.

East Pacific Green Sea Turtles (Chelonia
mydas)

The Mexican nesting stock of East
Pacific green sea turtles is currently
designated as endangered due to the decline in nesting in recent decades, as
well as intense poaching and harvesting during the middle of the century. This
stock of green turtles ranges from South America to southern
California, but the vast majority are born on sandy beaches in southern Mexico,
primarily in the state of Michoacán, before beginning their juvenile stage of
life (see region shown in map below).

The
oceanic juvenile stage of East Pacific green turtles remains one of the least
understood stages in the life history of this endangered species population. In
the last decade, progress has been made on elucidating the lost years oceanic
juvenile state of marine turtles (Reich et al.
2007, Arthur et al. 2008, Cardona et al. 2010, Snover et al. 2010).
However, the duration of the juvenile stage of this endangered stock of green
turtles remains to be known and is current research priority for the management
of this slowly recovering population of turtles.

This oceanic juvenile stage is currently
estimated to last from two to five+ years, during which turtles feed
opportunistically and omnivorously - eating what they can find - along ocean current boundaries. Eventually these juveniles are large enough settle into nearshore subtidal algae and seagrass
ecosystems to feed and grow (Carr 1987, Reich et al. 2007).
If they were to live in nearshore habitats too soon, they would be too small and would become prey to larger fish, birds and other common predators in coastal ecosystems. After growing even more in these nearshore habitats, turtles reach maturity, at around thirty years of age, and return to their nesting sites to breed approximately every three years. In
between breeding times, the turtles migrate between nesting grounds and
foraging areas in Mexico (both along the Pacific coast and in the Gulf of California) and some even live as far north as California, USA. Main foraging (or eating) grounds include bays
and lagoons where green turtles eat several species of eelgrass, marine algae,
and invertebrates like crabs and snails (Seminoff et al.
2002a, Lopez-Castro et al. 2010, Lemons et al. 2011).

Larger region, zoomed in area to the right -->

Region of East Pacific Green Turtles: San Diego shown to the north, nesting area in Michoacán in the south.

This population of green turtles is
considered a regional management unit - meaning it has its own management plans and and protection. It was also once the target of a large turtle
fishery that reached its peak in the 1970s. The fishery harvested turtle meat
and eggs that were sold and consumed in large numbers. Due to this fishery,
this once plentiful population of green turtles is currently a fraction of the size of what
it once was (National Marine Fisheries Service 2007a). Cooperative efforts,
including protective measures put in place in 1990 by the Mexican government,
prevented further collapse of the green turtle population, and evidence of slow
recovery continues to be observed (Koch et al.
2006). Despite international legal protection, impacts beyond poaching,
such as fishing bycatch, boat strikes, development of nesting beaches, and
development of coastal foraging areas, continue to threaten these sea turtles.

North Pacific Loggerhead Sea Turtles
(Caretta caretta)

In
the Pacific, two genetically distinct stocks of loggerhead turtles have been
identified (National Marine Fisheries Service
2007b). One in the South Pacific, which nests in and around Australia,
and one in the North Pacific, which nests on multiple islands of Japan (Bowen et al. 1995). Both of the Pacific
population stocks are currently designated as endangered due to the decline in
nesting in recent decades (Kamezaki et al., 2003). In the North Pacific, over
40 nesting beaches are known and monitored, yet approximately 30-40% of this
population’s nesting occurs at three main beaches on the small island of Yakushima
at the southern end of Japan (Kamezaki et al., 2003). Hatchlings from these
beaches, once they swim offshore, are moved north and then east by swimming with the Kuroshio Current and the North
Pacific drift - eventually being guided toward the Central North Pacific (CNP), close to Hawaii. Immature loggerhead
turtles have been found as fisheries bycatch in the CNP around the extended
Hawaiian Islands. Satellite tracks of juvenile loggerheads show extended and
concentrated foraging in the areas where currents help create highly productive waters (specifically near the Kuroshio Extension Bifurcation Region and the Transition Zone Chlorophyll Front) (Polovina et al. 2004, Polovina et al. 2006, Kobayashi et al. 2008).
Eventually some of these turtles are guided by the California Current, and a
large aggregation of loggerheads, estimated to be in the tens of thousands,
forage off the coast of the Baja California Peninsula (BCP) (Lewison et al. 2004, Wallace et al. 2008, Peckham et
al. 2011).

Loggerhead sea turtle. Image by author.

Studies
examining body size and stomach contents of fishery-bycaught and dead-stranded
turtles show that turtles near Hawaii (CNP turtles) are smaller in size and
foraged primarily on pelagic invertebrates and fish commonly associated with
fisheries bycatch and bait. In contrast, turtles examined from the Baja region
(BCP) were slightly larger in size, although still juveniles, and ate primarily
pelagic red crabs (Pleuroncodes planipes)
and fish associated with fishing operations (Peckham
et al. 2011). Upon reaching maturity, adult loggerheads migrate back across
the Pacific to return to the neritic waters of the Western Pacific where they
remain to forage and breed for the rest of their lives (Resendiz et al. 1998, Nichols et al. 2000, Hatase et al. 2002, Hatase et
al. 2004).

A
primary threat to the North Pacific loggerheads in the East Pacific is the
intense mortality associated with small-scale artisanal fisheries. Conservative
estimates indicate that 1000+ individual loggerheads are killed per year (Peckham et al. 2007, Peckham et al. 2008) from
bycatch related to BCP small-scale fisheries. Upwelling in the East Pacific creates
productive waters that result in large amounts of biomass (plankton, fish & other animals) in a concentrated
area. While the loggerheads in this area are foraging primarily on swarms of
red crab, many types of fish also forage in these productive waters. The
concentration of fish attracts local fishers to the same location, thereby
contributing to high turtle-fishery interaction making this area a bycatch
“hotspot” (Peckham et al. 2007, Wingfield et al.
2011).

Notice the concentration of loggerhead turtles (red areas) in the vicinity of small-scale fishing efforts (white boat logos and outline).

In
the past fifteen years, significant efforts have been made to engage local
fishers and address the problem of this turtle hotspot. Gradually, fishing
practices are being adjusted in order to reduce interactions with turtles and a
protected area has been established (Lewison et
al. 2011). Cooperative work by Grupo Tortuguero
and their partners in communities throughout Baja and Pacific Mexico
states have been addressing this issue of bycatch, as well as poaching
and other threats to marine turtles and sea life through diverse
community engagement programs. And while much has been learned about this population of
loggerheads and their use of this productive yet dangerous East Pacific
habitat, it remains unclear how long the juvenile loggerheads forage in this
BCP hotspot region prior to migrating to the West Pacific. Understanding the
duration of exposure to sources of high mortality is important information for
population management (Hamann et al. 2010,
Wallace et al. 2011).

My
research will address these some of these unknowns related to habitat use and duration of different life stages of turtles of each of these two
populations.

Me helping H. Peckham tag and measure a loggerhead in the "loggerhead hotspot" off of

About Me

Cali is a Postdoctoral Fellow at University of California, San Diego's Ecology program, having received her Ph.D. in Ecology, Behavior
& Evolution. She is a graduate of UCSD's Scripps Institution of Oceanography, where she received her Masters in Marine Biodiversity and Conservation. Her work conducting environmental monitoring in San Diego includes assisting NOAA researchers monitor the population of green sea turtles in the San Diego Bay. The work highlighted on this blog is in partnership with NOAA's Southwest Fisheries Science Center (see links for more information).